In traditional telephony, that is, circuit switched telephony, for a call to be established between two remote telephones, that is, telephones connected to different local exchanges, signalling is used to establish a path prior to establishing the call itself. The path in the above example comprises initiating telephone to its local exchange, initiating local exchange to trunk connection, trunk connection to receiving local exchange, and receiving local exchange to receiving telephone. Here, the signalling and the call usually take the same path and there is full control of the path through each element in the path. As there is full control, it is relatively straightforward to determine whether a call between two telephones can be established or not.
In conventional internet protocol (P) telephony, the local exchanges are replaced by local ‘gatekeepers’ which communicate with one or more trunk gatekeepers to establish the path between the initiating telephone and the receiving telephone. Here, signalling is effected through the trunk gatekeeper(s) but the call does not take the same path. In this case, the trunk gatekeeper(s) control the bandwidth which can be used in establishing the call, and if the bandwidth is not sufficient, the call is not established.
With the advent of opaque trunk IP telephony, there is no gatekeeper in the IP network which forms the “trunk”. As a result, there is effectively no control over being able to establish a call successfully. Here, the initiating telephone cannot be certain that a call, once established, will be successfully completed.